driver core: add TAINT_FORCED_BIND for when userspace manually messes with devices and drivers

The ability to add and remove devices from a driver through the sysfs
"bind" and "unbind" files was created all those decades ago as a way
that kernel developers can iterate faster, and provide a debugging way
for users to attempt to add a new device to a driver without having to
rebuild their kernel.

This api over the years has been abused and recently come under a major
fuzzing "attack" through tools like syzbot which decided that it would
attempt to just randomly bind any driver to any type of device, causing
loads of unneeded errors and pointless kernel patches to be generated by
unsuspecting new developers.

Handle all of this by adding a new taint flag, TAINT_FORCED_BIND, which
will be set on the driver if the bind/unbind sysfs files are ever
written to.  This lets kernel developers "know" that a user is
attempting to do something that is not normal, and as such, if the
kernel breaks they get to keep the shiny pieces laying around on the
floor.

The flag is 'Y' which was unused, and can remembered as the user is
"yeeting" the device being operated on here (thrown with force without
regard for the thing being thrown).

Note, the taint flag gets set _BEFORE_ the bind/unbind callback happens,
as many times crashes/oops/warnings/failures happen within the callback,
and the taint flag needs to be there to show what was being attempted.
If it were to be set after the callback happens, the oops report would
not properly reflect what foolishness was being attempted.

Fuzzing tools like syzbot, that doesn't have hand-crafted rules to keep
the tool from hitting bind/unbind, should be run with panic_on_taint
enabled so that they fall over and don't continue on, thinking that they
actually found a real issue.

Userspace operations that rely on the bind/unbind files to work around
the lack of will to upgrade a kernel image to a newer version with
proper support for new devices, or the lack of will to submit valid
device ids to driver authors, will still work properly, but now the
kernel will be flagged in a way that will show that perhaps those users
should reconsider their behavior and work to have the drivers properly
support these devices in a "native" manner.

Finally, the bind/unbind files can find real use-after-free issues with
some drivers by forcing the process to happen virtually without having
to rely on manual removal processes.  Those real bugs should still be
worked on, but by adding this taint flag, developers can more easily
determine bug reports that are actually worth looking at.

Reviewed-by: Johan Hovold <johan@kernel.org>
Tested-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
Reviewed-by: Bradley Morgan <brads@mainlining.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260914-bind_taint-v4-3-eadf8a090903@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/tainted-kernels.rst

@@ -79,9 +79,9 @@ which bits are set::
 Table for decoding tainted state
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-===  ===  ======  ========================================================
+===  ===  =======  ========================================================
 Bit  Log  Number   Reason that got the kernel tainted
-===  ===  ======  ========================================================
+===  ===  =======  ========================================================
   0  G/P        1  proprietary module was loaded
   1  _/F        2  module was force loaded
   2  _/S        4  kernel running on an out of specification system
@@ -102,7 +102,8 @@ Bit  Log  Number  Reason that got the kernel tainted
  17  _/T   131072  kernel was built with the struct randomization plugin
  18  _/N   262144  an in-kernel test has been run
  19  _/J   524288  userspace used a mutating debug operation in fwctl
-===  ===  ======  ========================================================
+ 20  _/Y  1048576  device was manually bound or unbound from a driver
+===  ===  =======  ========================================================
 
 Note: The character ``_`` is representing a blank in this table to make reading
 easier.
@@ -189,3 +190,6 @@ More detailed explanation for tainting
  19) ``J`` if userspace opened /dev/fwctl/* and performed a FWTCL_RPC_DEBUG_WRITE
      to use the devices debugging features. Device debugging features could
      cause the device to malfunction in undefined ways.
+
+ 20) ``Y`` If userspace wrote to the `bind` or `unbind` sysfs files and
+     successfully bound or removed a device from a driver.

drivers/base/bus.c

@@ -19,6 +19,7 @@
 #include <linux/string.h>
 #include <linux/mutex.h>
 #include <linux/sysfs.h>
+#include <linux/panic.h>
 #include "base.h"
 #include "power/power.h"
 
@@ -241,6 +242,7 @@ static ssize_t unbind_store(struct device_driver *drv, const char *buf,
 
 	dev = bus_find_device_by_name(bus, NULL, buf);
 	if (dev && dev->driver == drv) {
+		add_taint_module(drv->owner, TAINT_FORCED_BIND, LOCKDEP_STILL_OK);
 		device_driver_detach(dev);
 		err = count;
 	}
@@ -264,6 +266,7 @@ static ssize_t bind_store(struct device_driver *drv, const char *buf,
 
 	dev = bus_find_device_by_name(bus, NULL, buf);
 	if (dev && driver_match_device(drv, dev)) {
+		add_taint_module(drv->owner, TAINT_FORCED_BIND, LOCKDEP_STILL_OK);
 		err = device_driver_attach(drv, dev);
 		if (!err) {
 			/* success */

include/linux/panic.h

@@ -88,7 +88,8 @@ static inline void set_arch_panic_timeout(int timeout, int arch_default_timeout)
 #define TAINT_RANDSTRUCT		17
 #define TAINT_TEST			18
 #define TAINT_FWCTL			19
-#define TAINT_FLAGS_COUNT		20
+#define TAINT_FORCED_BIND		20
+#define TAINT_FLAGS_COUNT		21
 #define TAINT_FLAGS_MAX			((1UL << TAINT_FLAGS_COUNT) - 1)
 
 struct taint_flag {

include/trace/events/module.h

@@ -26,7 +26,8 @@ struct module;
 	{ (1UL << TAINT_OOT_MODULE),		"O" },		\
 	{ (1UL << TAINT_FORCED_MODULE),		"F" },		\
 	{ (1UL << TAINT_CRAP),			"C" },		\
-	{ (1UL << TAINT_UNSIGNED_MODULE),	"E" })
+	{ (1UL << TAINT_UNSIGNED_MODULE),	"E" },		\
+	{ (1UL << TAINT_FORCED_BIND),		"Y" })
 
 TRACE_EVENT(module_load,
 

kernel/panic.c

@@ -826,6 +826,7 @@ const struct taint_flag taint_flags[TAINT_FLAGS_COUNT] = {
 	TAINT_FLAG(RANDSTRUCT,			'T', ' '),
 	TAINT_FLAG(TEST,			'N', ' '),
 	TAINT_FLAG(FWCTL,			'J', ' '),
+	TAINT_FLAG(FORCED_BIND,			'Y', ' '),
 };
 
 #undef TAINT_FLAG
@@ -862,9 +863,9 @@ static void print_tainted_seq(struct seq_buf *s, bool verbose)
  * exact size is allocated dynamically; the initial buffer remains
  * as a fallback if allocation fails.
  *
- * The verbose taint string currently requires up to 327 characters.
+ * The verbose taint string currently requires up to 344 characters.
  */
-#define INIT_TAINT_BUF_MAX 350
+#define INIT_TAINT_BUF_MAX 370
 
 static char init_taint_buf[INIT_TAINT_BUF_MAX] __initdata;
 static char *taint_buf __refdata = init_taint_buf;

tools/debugging/kernel-chktaint

@@ -211,6 +211,14 @@ else
 	addout "J"
 	echo " * fwctl's mutating debug interface was used (#19)"
 fi
+
+T=`expr $T / 2`
+if [ `expr $T % 2` -eq 0 ]; then
+	addout " "
+else
+	addout "Y"
+	echo " * device was manually bound or unbound from a driver (#20)"
+fi
 echo "Raw taint value as int/string: $taint/'$out'"
 
 # report on any tainted loadable modules